1. Field of the Invention
The present invention relates to a device for position indication and the detection of guidance errors, including a scale and a position indication scanning unit as well as a guidance error scanning unit.
2. Discussion of Related Art
Occasionally there is a requirement in connection with modern precision machine tools or coordinate-measuring apparatus to detect, besides the exact indication of the position along a machine axis, possibly existing guidance errors of the respective axis. The detection by measurement techniques of one of the different guidance errors is also called straightness measuring. It is possible to computationally correct the respective measured position value during operation in a suitable evaluation device.
These problems are extensively discussed in EP 0 082 441 A1 and corresponding U.S. Pat. No. 4,587,622, the entire contents of which are incorporated herein by reference. FIGS. 3 and 4 of this document furthermore show a suitable device that makes it possible to determine the respective position along an axis by a high-resolution incremental measurement, as well as to perform the above mentioned detection of guidance errors, or also the straightness measurement. A scale is provided for this that, besides a position measuring graduation in the form of a customary incremental graduation in the measurement direction, also includes a guidance error measuring graduation, which is arranged vertically with respect to it. The guidance error measuring graduation includes graduation marks, which are oriented parallel with the measurement direction and extend over the corner area of a support body of square cross-section. Photoelectrical measuring systems are mentioned for scanning the various measuring graduations which, however, are not further discussed.
A further device for simultaneous position and guidance error measurement is known from EP 0 660 085 A1. Here, a measurement graduation is scanned by a CCD (charge coupled device) line, wherein the measurement graduation contains several tracks. Two tracks with bars, which are oriented parallel with the measurement direction, are arranged symmetrically adjoining a center non-periodic bar code structure. Scanning of the last mentioned tracks, which are oriented vertically with respect to the actual measurement direction, in turn allows the detection of possible guidance errors, or the straightness measurement. However, here the absolute position indication is performed with a relatively rough resolution, therefore this device is not suitable for high-precision applications.